Organic electroluminescent (OEL) device, alternately known as organic light emitting diode (OLED), is useful in flat-panel display applications. This light-emissive device is attractive because it can be designed to produce red, green, and blue colors with high luminance efficiency; it is operable with a low driving voltage of the order of a few volts and clearly viewable from oblique angles. These unique attributes are derived from a basic OLED structure comprising of a multilayer stack of thin films of small-molecule organic materials sandwiched between an anode and a cathode. Tang et al in commonly-assigned U.S. Pat. Nos. 4,769,292 and 4,885,211 have disclosed such a structure. The common electroluminescent (EL) medium is comprised of a bilayer structure of a hole-transport layer (HTL) and an electron-transport layer (ETL), typically of the order of a few tens of nanometer (nm) in thickness for each layer. When an electrical potential difference is applied at the electrodes, the injected carriers—hole at the anode and electron at the cathode—migrate towards each other through the EL medium and a fraction of them recombine to emit light. The intensity of electroluminescence is dependent on the EL medium, drive voltage, and charge injecting nature of the electrodes. The light viewable outside of the device is further dependent on the design of the organic stack and optical properties of the substrate, anode and the cathode.
Conventional OLEDs are bottom emitting (BE), meaning that the display is viewed through the substrate that supports the OLED structure. The devices normally employ glass or other transparent substrates having a layer of transparent anode layer, generally of indium-tin-oxide (ITO). The cathode is typically an opaque and reflective thin film of MgAg, although lithium-containing alloys are also known to provide efficient electron injection. The light generated within the device is emitted in all directions. About 80% of generated light is not available for viewing due to losses in wave-guiding modes in glass, ITO and organic layers. The light emitted toward the anode at less than the critical angle passes through the anode and the substrate to the viewer, and the light emitted in the opposite direction is reflected at the cathode and passes through the substrate, enhancing the viewing intensity. A high-transparency substrate and anode and a high-reflectivity cathode are thus preferred.
The OLEDs are typically coupled with active matrix (AM) circuitry in order to produce high performance displays. In BE OLEDs, the circuitry (bus metals, thin film transistors (TFTs), and capacitors) is competing with pixel-emitting areas for space in the substrate. For these displays employing complex circuitry in the backplane the open area through which the light emerges is reduced. The ratio of the open area to that of the entire device area is called the aperture ratio. Due to reduction of the aperture ratio the display will run dim. To compensate for the reduced average brightness level, the drive current is increased subjecting the display to increased risk of operational degradation. It follows that more complex pixel drive circuitry cannot be readily implemented without further compromising aperture ratio and operation stability.
In top-emitting design the drive circuitry is fabricated on substrate and the light emits from the opposite surface. This design permits the use complex circuitry occupying whatever substrate space is needed and the aperture ratio is not affected. The high aperture ratio makes the display viewable consuming less power. The devices employing opaque backplanes such as silicon the OLED must be of the top-emitting type.
The top-emitting devices with inherently high aperture ratio are power efficient. But any device design should also be aimed at achieving highest possible efficiency. These devices have the prospect of running at low drive current while maintaining readability and thus extending the operational life. However, realizing high efficiency by reclaiming light lost to waveguiding modes can be very difficult. To recover even a fraction of light the device architecture can be very complex.
An approach to enhance the efficiency without introducing such complexity is to implement the microcavity design of the device structure as has been described. By employing a microcavity structure including highly reflective electrodes it is possible to significantly increase the out coupling of generated light. In the microcavity device the light emitted out of the cavity depends on the cavity design. The resonance wavelength of a microcavity is given by2Σ(nidi)/λ−(Φ1+Φ2)/360°=m,where m=0, 1, 2, . . . , λ is the peak wavelength of the light emerging out of the cavity consisting of layers having the thickness di and refractive index ni, and Φ1 and Φ2 are the phase shifts of light in degrees upon reflection from the two reflecting electrodes. The quantity nidi is conventionally called the “optical path length” in the material, so Σ(nidi) is the total optical path length in the microcavity. For a fixed optical path length of the microcavity, the strength of this emission (and to a much lesser degree its wavelength) is also influenced by the location of the emission zone within the cavity. If a reasonable distance between the cathode and HTL/ETL interface is selected, then the HTL thickness can be varied in order to vary the optical path length of the microcavity. The maximum luminance occurs for an HTL thickness such that the resonance wavelength of the microcavity is well-aligned with the peak in the innate emission spectrum of the particular dopant-host material. The first maximum occurs at the HTL thickness corresponding to m=0, and subsequent maxima occur at the HTL thickness corresponding to m=1, 2 and so on.
FIG. 1 is a plot of luminance of a top-emitting microcavity structure as a function of NPB HTL thickness up to the thickness corresponding to m=1 as determined by optical modeling. The structure of the OLED is: Glass/80 nm Ag/variable NPB/60 nm Alq/14 nm MgAg and includes the fully reflective Ag anode deposited on the glass substrate. The MgAg top electrode is the cathode, and being thin and semitransparent allows significant emission through the top surface of the OLED.
The FIG. 1 shows that for this structure the first maximum of the luminance occurs at 46 nm and the second maximum occurs at 196 nm of NPB thickness. It is obvious that such plots for the emission at other wavelengths will be similar to the FIG. 1 but the cavity length will be slightly different, and the maxima of resonance wavelengths will occur at slightly different NPB thicknesses. It has been found, however, that the OLED based on the above layer structure was inefficient. An interlayer between the Ag anode and the NPB HTL was found useful in lowering the operating voltage and improving the stability of the diode. The interlayer, including a fluorinated carbon or an oxide, is very thin (1 to 2 nm thick) and highly transparent. It thus does not significantly affect the optics of the OLED structure. The OLED structure with 46 nm NPB is most desirable for maximizing efficiency and minimizing the angular dependence of the color but the one with the 196 nm NPB can be desirable from a manufacturing point of view. This is because the thin NPB can yield shorted OLEDs or OLEDs that can catastrophically fail in use. On the other hand, a thick NPB HTL is likely to increase the drive voltage. It has been found, however, that the drive voltage of OLEDs having a MoOx HIL on an Ag anode and 200 nm NPB HTL is higher than can be accounted for solely by increased NPB thickness.